In recent years, a thin film transistor (TFT) in which a transparent conductive oxide polycrystalline thin film containing ZnO as a main ingredient is used as a channel layer has been actively developed (see Japanese Patent Application Laid-Open No. 2002-76356 and U.S. Publication No. 2006/0244107 A1).
The thin film can be formed at low temperature and is transparent to visible light, so it is possible to form a flexible transparent TFT on a substrate such as a plastic plate or a film.
Further, it is disclosed that thin film transistors having a transparent semiconductor using ZnO or the like form a matrix display apparatus (U.S. Pat. No. 6,563,174).
In U.S. Pat. No. 6,563,174, it is disclosed that a source electrode and a drain electrode on an oxide semiconductor can be formed by dry etching.
According to the article in Nature, 488, 432 (2004), there is disclosed a technology of using a transparent amorphous oxide semiconductor film (a-IGZO) containing indium, gallium, zinc, and oxygen as the channel layer of a TFT.
Further, it is described that a flexible and transparent TFT with a satisfactory field-effect mobility of 6 to 9 cm2V−1 s−1 can be formed on a substrate such as a polyethylene terephthalate (PET) film at room temperature.
Further, in Table 2 on page 74 of February 2006 NIKKEI MICRODEVICES, it is described that the operation was recognized as a flexible electronic paper using a thin film transistor where a-IGZO is used as a channel layer of the TFT.
Further, as a method of forming source/drain electrodes on a semiconductor layer, a structure where an etching stopper layer is provided on a semiconductor layer in a bottom gate type thin film TFT to lower leakage current between a source electrode and a drain electrode is disclosed (U.S. Pat. No. 5,403,755 and U.S. Publication No. 2006/0054888 A1).
U.S. Pat. No. 6,563,174 discloses that, in a bottom gate type thin film TFT having a transparent oxide semiconductor whose main component is ZnO, a source electrode and a drain electrode on an oxide semiconductor can be formed by dry etching. U.S. Pat. No. 6,563,174 also describes that a protective film which is practically essential in a bottom gate type thin film TFT is formed by a plasma CVD (P-CVD) method as a silicon nitride thin film.
In most of the related art except U.S. Pat. No. 6,563,174, a source electrode and a drain electrode are formed by a lift-off technique.
In lift-off, a problem arises such as readherence of particles of a lifted-off electrode film, thus it is difficult to produce a large-area TFT with high yields.
It is difficult to form a source electrode and a drain electrode by wet etching using an acid in designing a TFT, because even if an electrode material is a metal or a transparent oxide conductor, an oxide semiconductor whose main component is ZnO is susceptible to an acid and the etching speed is high.
In effect, a source electrode and a drain electrode are formed only by a dry etching process.
However, in an oxide semiconductor whose main component is ZnO, an oxygen vacancy is liable to occur and many carrier electrons are liable to be generated, thus an oxide semiconductor layer may be damaged in a process of etching a source electrode and a drain electrode.
There is also a method of providing a protective layer as an etching stopper layer in order to decrease damage to the semiconductor layer caused by the etching, but, even if such a protective film is formed, the oxide semiconductor layer is damaged and an OFF-current becomes larger.
Thus, there is a problem in that TFT characteristics with a satisfactory on/off ratio are difficult to realize with stability.